In a magnetic recording medium, a backing layer is generally provided on the surface of a support which is opposite to the surface provided with a magnetic layer in order to improve running stability at rewinding at a high speed rate, and to reduce the wear out of the support. It is known to add electroconductive substances such as carbon black into the backing layer to reduce occurrences of drop outs. The reason for the above addition of the electroconductive substances is to increase electroconductivity (to decrease surface electric resistance) of the magnetic recording medium and to remove the electric charge, whereby dust and contaminants can be prevented from adhering to the magnetic recording medium, and occurrences of drop out can be decreased. However, the above addition of electroconductive substances alone will not achieve sufficient antistatic effects, and prevention of increase of drop outs is also limited.
As an approach for improvement, it has been proposed to add a large supplemental amount of electroconductive substances such as carbon black into the magnetic layer to improve electroconductivity to prevent electrical charging of the magnetic recording medium as described, e.g., in JP-A-58-133626, JP-A-58-158032, JP-A-59-193533. (The term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, when the amount of fillers such as carbon black is increased in the magnetic layer to increase electroconductivity, dispersibility of the magnetic coating composition deteriorates, the surface properties of the magnetic layer deteriorate, or the amount of the magnetic particles in the magnetic layer decreases, whereby desirable electromagnetic characteristics (such as S/N characteristics or drop out) deteriorate. Therefore a satisfactory magnetic recording medium cannot be obtained.
As a result of thorough research by the inventors to solve the above-described problem of enhancing S/N characteristics while preventing drop out, a problem which cannot be solved by conventional technologies, it was found that the above problem can be solved by incorporating into at least one of the binders used in the magnetic layer and the backing layer, vinyl chloride type polymer resins having a unit of vinyl chloride in an amount of 70 wt% or more, a nitrogen content of from 0.01 to 1 wt% and a degree of polymerization of from 250 to 700.
However, the above method has not been sufficiently satisfactory when extremely fine magnetic particles which are difficult to uniformly disperse are used.